It has been known that BN ceramics are materials having very excellent antiwetting properties against melts of metal, glass and the like. Though BN ceramics having excellent antiwetting properties must be commonly used as materials for a furnace, in practice there are a number of reasons for preventing their use. A first reason is because when a member formed of the heretofore known BN ceramics comes into contact with melts of metal, glass and the like, it would be readily damaged by fusing. A second reason is because a member produced by sintering the heretofore known ceramics at the atmospheric pressure is quite indurable in use because of too low strength, hence it is a common practice to manufacture a BN ceramics member through a hotpress, and its manufacturing cost becomes high.
On the other hand, in order to continuously measure a temperature of molten metal within a molten metal container, it is a common practice to rely upon a thermocouple immersed in the molten metal. The thermocouple is inserted into a protective tube and it is put into the molten metal jointly with the protective tube. However, in connection to the use of the protective tube, there are the following problems:
1 Since the protective tube in the prior art put in molten metal at a high temperature such as that of molten steel is liable to be eroded, continuous temperature measurement is difficult.
2 In the conventional case, a protective tube is put into molten metal from its surface, and is held immovable by a holding member. According to this procedure in the prior art, the length of a protective tube becomes large, and its wall thickness must be made sufficiently large so that the protective tube may not be broken by flow of the molten metal. Consequently, there is a shortcoming that the protective tube becomes extremely expensive. In addition, a mechanism for holding and fixing the protective tube is complicated and handling thereof is intricate.